Multicarrier modulation techniques, such as orthogonal frequency-division multiplexing (OFDM) and discrete multi-tone (DMT), are now widely used for high-speed communications over bandpass communications channels. Examples of their use include the broadcasting of digital audio and video. They further include implementations of the physical layer for wireless networking standards, e.g., Institute of Electrical and Electronics Engineers (IEEE) standards 802.11a, g, j, n, and 802.16.
OFDM and DMT systems are merely two examples of multiple-carrier wireless communication systems utilizing multiplexing techniques. Such multi-carrier systems employ multiple-access techniques to transmit content across a wireless communications channel comprising a number (greater than one) of sub-carriers.
An OFDM system uses several low-rate sub-carriers to transmit data. The frequencies of the sub-carriers and the symbol period are chosen so that the sub-carriers are orthogonal over the symbol period and do not interfere with one another. In an OFDM system, the data is split into N streams, which are then used to independently modulate closely-spaced sub-carrier frequencies or tones in parallel. In the applications mentioned above, N is typically 64 or more. The frequency separation between the sub-carriers is 1/T, where T is the OFDM symbol time duration.
Practical systems use an inverse-discrete-Fourier transform (implemented as an inverse Fast Fourier Transform or IFFT) to generate a sampled version of a composite time-domain signal that can be converted to a signal suitable for radio transmission. The most distinct advantage of OFDM over single-carrier modulation is the longer symbol period for a given data rate, which inherently mitigates inter-symbol interference in time-dispersive channels without having to resort to elaborate equalization techniques.
However, because an OFDM signal is the sum of multiple sinusoidal waves, high-amplitude peaks in the composite time-domain signal occur when the signals from different tones add constructively. Compared with the average signal power, the instantaneous power of these peaks is high, which means that the signal has a high peak-to-average power ratio (PAPR). The occurrence of these peaks may seriously hamper practical implementations and is generally considered to be a significant challenge of OFDM.
In order to transmit an OFDM signal, the energy of the time-domain signal must be increased by a power amplifier. However, a high PAPR means that the OFDM signal is sensitive to any non-linearities exhibited by the transfer characteristic of the power amplifier. It is possible to reduce the PAPR by using special coding techniques for encoding the data that result in OFDM signals with a low PAPR, but this does not eliminate the problem.